I am thinking there is something about the fabric of the sofa, either the taste or the feel of it on her tongue that is causing your kittys licking. I always keep a fresh
clean sheet draped over my fabric
couch for easier cleaning. Instead of scrubbing fabric couch cushions or backs, I just take the full size sheet off and pop it into the washer once or twice a week.
I hope there are more members like Opus88 who might have a few insights into this brand of cat behavior....